US 20100172339 A1 Abstract A method and system improves two-way radio ranging accuracy by estimating a relative clock frequency offset between a first clock X of a first transceiver and a second clock Y a second transceiver. The first transceiver transmits a first packet at time t
_{0 }received by the second transceiver at a time t_{1}. The second transceiver transmits a second packet at a time t_{2 }received by first transceiver at a time t_{3}. The second transceiver transmits a third packet at a time t_{4 }received at a time t_{5}. The relative clock frequency offset is thenwhere f is a nominal clock frequency of the first and second clocks, N
^{Y} _{24 }is a measured first delay between times t_{2 }and t_{4 }of the second clock, N^{X} _{35 }is a measured second delay between times t_{3 }and t_{5 }of the first clock X.Claims(11) 1. A method for improving two-way radio ranging accuracy by estimating a relative clock frequency offset between a first clock X of a first transceiver and a second clock Y a second transceiver, comprising:
transmitting, by the first transceiver, a first packet at time t _{0 }of the first clock X;receiving, by the second transceiver, the first packet at a time t _{1 }of the second clock Y;transmitting, by the second transceiver, a second packet at a time t _{2 }of the second clock;receiving, by the first transceiver, the second packet at a time t _{3 }of the first clock X;transmitting, by the second transceiver, a third packet at a time t _{4 }of the second clock Y; andreceiving, by the first transceiver, the third packet at a time t _{5 }of the first clock X, and in which the relative clock frequency offset iswhere f is a nominal clock frequency of the first and second clocks, N
^{Y} _{24 }is a measured first delay between times t_{2 }and t_{4 }of the second clock, N^{X} _{35 }is a measured second delay between times t_{3 }and t_{5 }of the first clock X.2. The method of claim :1, further comprising:
measuring a distance between the first transceiver and the second transceiver based on the relative clock frequency offset. 3. The method of 4. The method of 5. The method of 6. The method of 7. The method of 8. The method of canceling an effect of channel impulse response, RF circuit and baseband circuit imperfection while determining the relative clock frequency offset. 9. The method of increasing the first delay to reduce quantization errors. 10. The method of in which t
^{X} _{03}=t_{3}−t_{o }measured by the first clock X, and t^{Y} _{12}=t_{2}−t_{1 }measured by clock Y.11. A system for improving two-way radio ranging accuracy by estimating a relative clock frequency offset between a first clock X of a first transceiver and a second clock Y a second transceiver, comprising:
a first transceiver transmits a first packet at time t _{0 }of a first clock X;a second transceiver receives the first packet at a time t _{1 }of a second clock Y, and in which the second transceiver transmits a second packet at a time t_{2 }of the second clock, which is received by the first transceiver at a time t_{3 }of the first clock X, and in which the second transceiver transmit a third packet at a time t_{4 }of the second clock Y, which is received by the first transceiver at a time t_{5 }of the first clock X, and in which the relative clock frequency offset iswhere f is a nominal clock frequency of the first and second clocks, N
^{Y} _{24 }is a measured first delay between times t_{2 }and t_{4 }of the second clock, N^{X} _{35 }is a measured second delay between times t_{3 }and t_{5 }of the first clock X. Description This invention relates to wireless communication systems, and more particularly, to accurate ranging estimation through relative clock frequency offsets (RCM) compensation between wireless devices. In many wireless communication networks, devices are not synchronized to a single reference clock. Instead, each device relies on its own clock. Due to the limitation of the hardware, manufacturing processes, temperature drift, component aging, etc., the actual operating frequency of the clock is generally different from the designed nominal frequency. For example, if a 100 MHz clock signal is generated by a crystal that has a tolerance of +/−20 parts per million (ppm), the actual frequency of the clock can be any value in a range from 99,998,000 Hz to 100,002,000 Hz. A difference between the actual frequency of the clock and the designed nominal frequency is referred to as the ‘clock frequency offset’ (CFO) or ‘absolute clock frequency offset’ (ACFO). The difference between the frequencies of two independent clocks (with the same nominal frequency) is called ‘relative clock frequency offset’ (RCFO). For example, for two clocks with nominal frequency of 100 MHz, if one clock has a frequency of 99,998,000 Hz and the other clock has a frequency of 100,002,000 Hz. The RCFO is 4 KHz, or 40 ppm. The CFO and RCFO can cause problem. When the clock is used for time measurement, the ACFO and/or RCFO introduce errors. In digital circuit, time is generally measured as the number of clock cycles between two time instances
where T
where T If the same time period is measured by two devices using independent clocks, the RCFO causes a discrepancy in the measurements. For example, if the nominal clock frequency is 100 MHz for both timer X and timer Y, timer X has a −20 ppm offset and timer Y has a +20 ppm offset, which is a RCFO of 40 ppm. A 1 msec time period will be measured as 99,998,000 ns and 100,002,000 ns respectively for timer X and timer Y, a difference of 40 ns. The RCFO has a significant impact on the ranging accuracy of a two-way TOA (TW-TOA) system. A TW-TOA is a method can also be used for ranging between two transceivers. TW-TOA does not require exact synchronization between the transmitter and receiver. The TW-TOA ranging method is used in the IEEE 802.15.4a standard. In TW-TOA, two transceivers exchange packets and the round trip delay is measured. A typical exchange is as follows: the first device transmits a packet to the second device. After receiving the packet, the second device transmits a packet back to the first device. The first device measures the total time elapsed from the transmission of the first packet to the reception of the second packet. The second device measures the time elapsed from the reception of the first packet to the transmission of the second packet. The round trip travel time of the signal is calculated as the difference between these two measured values. The distance between the devices is calculated as the product of one half of the round trip traveling time multiplied by the speed of the signal (3×10 Because the turnaround time at the second device is much larger than the signal ‘flying’ time, the error caused by RCFO is the dominant factor in the overall time estimation error for a TW-TOA system. If the devices can obtain an accurate estimation of the RCFO, then the time estimation accuracy can be improved by compensating for it. Clearly, an accurate RCFO estimation is very important in achieving accurate time measurement. Let t and t The range between device X and Y is where C is the speed of the electromagnetic signals, e.g., 3×10 Because the true round-trip fly time is the estimation error is Given that the process time is much longer than the fly time and t where Δf The process time, t Conventional methods for estimating the RCF(in wireless systems include the following. One method uses the preamble of a received packet. Generally the preamble of the packet includes multiple symbols. By measuring a length of the preamble or a time interval between two symbols, the receiving device can estimate the RCFO of its clock with respect to the clock of the transmitting device. Such a method does not yield an accurate RCFO estimation because the preamble has a finite duration. Therefore, the time interval between two symbols cannot be very large, the method does not benefit from the processing gain by using as many symbols in the preamble, and the method is sensitive to carrier frequency offset and sample timing errors. A symmetric double-sided two-way protocol exchange data/ranging packet between two devices in the following sequence: -
- Device A transmits a first packet;
- Device B receives the first packet;
- Device B transmits a second packet;
- Device A receives the second packet;
- Device A transmits a third packet; and
- Device B receives the third packet.
- Devices A and B exchange the timing information in additional data packets.
Device A measures a round trip time t The drawbacks of such a method are: the times t Another method uses a combination of preamble and payload section or possibly post amble, see U.S. patent application Ser. No. 11/749,517, “Method for Reducing Radio Ranging Errors Due to Clock Frequency Offsets,” filed by Sahinoglu on Jun. 30, 2007, incorporated here in by reference. Another method uses frequency domain analysis. By performing a fast Fourier transform (FFT) on part or all of the preamble of a packet, the RCFO can be estimated. This approach requires the storage of a large number of samples, a complex FFT operation and therefore requires additional hardware, software and power. The accuracy is dependent on the signal-to-noise ratio (SNR) of the received signal and the size of the preamble portion used for FFT. It is desired to provide a method for estimating the RCFO without the above problems and complexities. The invention provides a method for estimating a relative clock frequency offset (RCFQ) between two wireless communication devices with asynchronous clocks by performing a data packet exchange. The method minimizes range estimation error in a two-way time-of-arrival (TOA) measurements. A first transceiver transmits a first packet. A second transceiver transmits a second packet, and then, after a first delay a third packet. The first transceiver receives the second packet, and then after second delay the third packet. The relative clock frequency offset is determined from the measured first and second delays. Wireless Ranging Devices Timers The timer starts counting when a signal cnt Relative Clock Frequency Offsets (RFCO) If the nominal clock frequency of the timer is f
where Δf If N
If N
RCFO Protocol At time t After receiving the ranging request packet and a delay of t After receiving the ranging request packet. The estimated round trip time without considering the frequency offsets is where, t
and t
The range or distance
where C is the speed of light. Because the true round trip time is the estimation error is Given that the processing time is much longer than the signal ‘flying’ time, i.e.,
where Δf However, in practice, the desired improvements may not be feasible. The value Δf The processing time t However, if we obtain an accurate estimation of Δf
The estimation error is
where Δf′ Equation (3) shows that an accurate estimation of relative frequency offset reduces the time estimation error significantly. If an accurate relative frequency clock offset estimation is achieved, i.e., Δf′ To estimate the RCFO, device X measures the second delay t Device Y transmits the time measurement N Device X determines the relative clock frequency offset (RICO) Δf
where f is the nominal frequencies of the first and second clocks. The first delay N Then, device X can then determine t The value of the first delay N -
- N
^{Y}_{24 }is predetermined by the network; - N
^{Y}_{24 }is predetermined by device Y and transmitted in the ranging acknowledgement packet**112**; - N
^{Y}_{24 }is transmitted in the ranging data packet**113**; and - N
^{Y}_{24 }is transmitted to device X in some other packet.
- N
It should be understood, the device Y can also determine the RCFO similarly provided with the second delay. In the cases when the value of RCFO Δf Compared with conventional method, the invention has the following advantages. The invention does not require a modification of a structure of packets that are used to perform the ranging or data communication. Any effect of channel impulse response, RF circuit and baseband circuit imperfection are cancelled. Any quantization error caused by a finite period of the clock can be reduced by increasing the first delay. The accuracy of estimating the RCM is high. The requirement for additional hardware, computation and power is very low. The method is compliant with the IEEE 802.15.4a standard, and does not require a modification to the media access (MAC) protocol used by that standard. Although the invention has been described by way of examples of preferred embodiments, it is to be understood that various other adaptations and modifications can be made within the spirit and scope of the invention. Therefore, it is the object of the appended claims to cover all such variations and modifications as come within the true spirit and scope of the invention. Referenced by
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